Liquid developing process in an electrostatographic imaging system

ABSTRACT

An electrostatographic imaging system employing liquid development of an electrostatic latent image present on an electrophotographic sheet wherein prior to development at least the nonimage bearing surface of said sheet is contacted with an insulating liquid having a boiling point not exceeding 100* C. and subsequently both surfaces of said sheet are contacted with an insulating liquid having a boiling point greater than about 150* C. The electrophotographic sheet is dried immediately after immersion in the liquid developer.

United States Patent Sato et al.

[54] LIQUID DEVELOPING PROCESS IN AN ELECTROSTATOGRAPI-IIC IMAGINGSYSTEM [72] inventors: Masamichi Sato; Osamu Fukushima, both of Asaka,Japan [73] Assignee: Xerox Corporation, Stamford, Conn.

[22] Filed: Sept. 8, 1970 [211 App]. No.: 70,551

[30] Foreign Application Priority Data Sept. 19, 1969 Japan ..44/74406[52] U.S. Cl. ..96/l.8, 96/1 R, 96/1 LY, 117/37 LE [51] Int. Cl. ..G03g5/00, 603g 7/00 [58] Field of Search ..96/1, 1 LY; 117/37 LE [56]References Cited UNITED STATES PATENTS 3,512,965 4/1970 Matkam..... .g

[451 Feb. 15, 1972 3,540,885 11/1970 Honjo et al. ..96/l

Primary Examiner-George F. Lesmes Assistant Examiner-John R. MillerAtt0rney.lames .l. Ralabate, Albert A. Mahassel and Samuel E. MottABSTRACT 9 Claims, N0 Drawings LIQUID DEVELOPING PROCESS IN ANELECTROSTATOGRAPHIC IMAGING SYSTEM BACKGROUND OF THE lNVENTlON Thisinvention relates to imaging systems and more particularly, to liquiddevelopment systems for developing electrostatic latent images presenton electrophotographic layers.

The formation and development of images on the surface of photoconductormaterial by electrostatic means is well known. The basic xerographicprocess as taught by C. F. Carlson in U.S. Pat. No. 2,297,691 involvesplacing a uniform electrostatic charge on a photoconductive insulatinglayer exposing the layer to a light and shadow image to dissipate thecharge on the areas of the layer. exposed to the light and developingthe resulting electrostatic latent image by depositing on the image afinely divided electroscopic marking material referred to in the art astoner". The toner will normally be attracted to those areas of the layerwhich retain a charge thereby forming a toner image corresponding to theelectrostatic latent image. The powder image may then be transferred toa support surface such as paper and permanently affixed to the supportby any suitable means such as heat fixing or solvent fixing.Alternatively, the powder image may be fixed to the photoconductivelayer if elimination of the powder transfer step is desired. Inaddition, instead of latent image formation by uniform charging followedby imagewise exposure, the latent image may be formed by directlycharging the layer in image configuration. Other methods are known forapplying electroscopic particles to the imaging surface. Ineluded withinthis group are the cascade development technique disclosed by E. N. Wisein U.S. Pat. No. 2,618,552; the powder cloud development techniquedisclosed by C. F. Carlson in U.S. Pat. No. 2,221,776; and the magneticbrush process disclosed for example, in U.S. Pat. No. 2,874,063.

Development of an electrostatic latent image may also be achieved withliquid rather than dry developer materials. In conventional liquiddevelopment, more commonly referred to as electrophoretic development,an insulating liquid vehicle having finely divided solid materialdispersed therein contacts the imaging surface in both charged anduncharged areas. Under the influence of the electric field associatedwith a charged image pattern, the suspended particles migrate toward thecharged portions of the imaging surface separating out of the insulatingliquid. This electrophoretic migration of charged particles results inthe deposition of the charged particles on the imaging surface in imageconfiguration. Electrophoretic development of an electrostatic latentimage may, for example, be obtained by pouring the developer over theimage bearing surface, by immersing the imaging surface in a pool of thedeveloper or by presenting the liquid developer on a smooth surfaceroller and moving the roller against the imaging surface. The liquiddevelopment technique has been shown to provide developed images ofexcellent quality and to provide particular advantages over otherdevelopment methods in offering ease in handling.

Automatic copying machines employing liquid development techniquesparticularly with electrophotographic sheets as the imaging surface arein widespread use today. In a particularly successful machine concept,the electrophotographic sheet bearing an electrostatic latent image isimmersed in the liquid developer so that both the image bearing surfaceand the rear surface are contacted with the liquid developer. Tonerparticles present in the developer liquid which are very close to thesurface of the electrophotographic sheet deposit on the sheetirrespective of the presence or absence of electrical charge on both theimage bearing surface and the rear surface and may remain permanently onthe surfaces. This small deposition of toner particles in the backgroundareas on the electrophotographic sheet is referred to in the art asfogging. Fogging also occurs by the deposition of toner particles fromthat portion of the liquid developer which penetrates into thephotoconductive layer and the base of the electrophotographic sheet.Fogging due to penetration by this mechanism is particularly conspicuouswhen employing an electrophotographic sheet such as Electrofax paper,which is a paper base coated with a mixture of a photoconductive powderin an insulating resin.

To minimize or eliminate the occurrence of fogging, prior to immersingthe electrophotographic sheet in the liquid developer, theelectrophotographic sheet may be prebathed or presoaked in a prebathingliquid which is retained on both sides of the sheet and in the fibrousconstruction of the base paper. The prebathing liquid is generallyinsulating, inert to the photoconductive material and miscible with theliquid developer in order to minimize any possibility of destruction ofthe imaging surface or contamination of the liquid developer. in orderto provide these requirements, the prebathing liquid is typically verysimilar or identical to the carrier liquid employed in the liquiddeveloper. Typically, these materials are nonpolar hydrocarbons andmineral oils. ln automatic copying machines, insulating, high-boilingliquids such as kerosene and Decalin have proved effective because theyare less susceptible to loss due to vaporization within the copyingmachine and consequently provide reduced loss over a period of time andalso provide little hazard due to fire.

Use of high-boiling materials as the prebathing liquid, however, hasseveral shortcomings. For example, when the electrophotographic sheethas a paper base, the prebathing liquid wets both the surface bearingthe electrostatic latent image and simultaneously penetrates into thefibrous construction of the paper base through the opposite surface ofthe sheet. Since the openings in the fibrous construction of the paperare extremely small, the prebathing liquid which has penetrated intothese openings cannot be very easily forced out. Thus, throughout thestage of development, the prebathing liquid, which is retained in thefibrous construction of the paper base intercepts the liquid developingagent and consequently prevents it from penetrating into the openings inthe fiber and smearing the base of the sheet. After the developmentstep, the eleetrophotographic sheet is passed through a pair ofsqueezing rollers to remove any liquid developer that may adhere to theimage bearing surface and the reverse surface of the sheet. However,this squeezing operation cannot remove the prebathing liquid which hasbeen lodged deep within the fibrous construction of the paper.Consequently, the sheet thus obtained has a large volume of prebathingliquid penetrated inside the paper serving as the base for theelectrophotographic sheet and therefore has a heavy, moist feeling. As aresult, it is not desirable to have this sheet placed on top of anothersheet because the prebath which has been retained within the sheet,gradually finds its way to the surface and wets the paper placed nearbyor may dissolve the printing ink on the printed sheet of paper whenplaced adjacent to it. To prevent this undesirable contamination ofadjacent surface, the prebathing liquid lodged deep within theconstruction of the paper base may be vaporized by means of a current ofwarm air. However, since the prebathing liquid has a high boiling point,this drying operation involving the use of a current of warrn air, whichmust be of such a temperature as to have no adverse affect on the paper,becomes excessively time consuming.

The practice of washing the electrophotographic sheet after developmentwith a cleaning liquid to remove liquid developer from theelectrophotographic sheet, prior to the removal of liquid developerthrough squeezing rollers is also frequently employed. It is generallypreferred to employ a cleaning liquid which is insulating and has a lowboiling point to take advantage of the comparative ease in drying. Thisis necessary because the cleaning liquid partially replaces theprebathing liquid which has been lodged deep within the construction ofthe paper and the low boiling cleaning liquid which has penetrated intothe paper construction may be readily vaporized with a current of warmair. If, however, the prebathing liquid which is lodged deep in thepaper is to be completely replaced with the cleaning liquid by thiswashing operation a considerable effort in time is necessary to completethis replacement. Therefore, any advantage that one would expect to bederived from the use of such a cleaning liquid is impractical in thecase of a treatment which necessarily must be carried out by anautomatic device requiring expeditious processing.

When a liquid of a relatively low boiling point is employed as theprebathing liquid, drying of the electrophotographic sheet afterdevelopment may be more rapidly accomplished. However, as alreadydiscussed above, the use of such low-boiling liquids provides variousdifficulties.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide a liquid development system which overcomes the above-noteddeficiencies.

It is another object of this invention to provide a liquid developmentsystem capable of producing dry copies at increased processing speed.

It is another object of this invention to provide a liquid developmenttechnique wherein the final copies are substantially free of prebathingliquid.

It is another object of this invention to provide an improved techniquefor prebathing an electrophotographic sheet prior to development.

It is another object of this invention to provide a liquid developmenttechnique producing developed images with reduced fogging in backgroundareas.

The above objects and others are accomplished, generally speaking, byproviding an electrostatographic imaging system of the liquiddevelopment type wherein prior to development of an electrostatic latentimage present on electrophotographic sheet with a liquid developerhaving charged particles suspended in an insulating liquid at least thereverse or nonimage bearing surface of the electrophotographic sheet iscontacted with a low-boiling, insulating liquid and subsequently, bothsurfaces of the electrophotographic sheet are brought into contact witha high-boiling insulating liquid.

More specifically, the present invention is based upon the principlethat the electrophotographic sheet is first wetted with a low-boilinginsulating liquid, hereinafter referred to as the first prebathingliquid, to allow it to penetrate sufiiciently into the paper serving asthe base of the electrophotographic sheet, and the electrophotographicsheet is subsequently contacted with a high-boiling insulating liquid,hereinafter referred to as a second prebathing liquid to permit theformation of a film of the second prebathing liquid on the imagingsurface. Thereafter, the electrophotographic sheet is immersed in theliquid developer to provide development of the electrostatic latentimage. Since the first prebathing liquid sufficiently penetrates thefibrous construction of the paper, the second prebathing liquid cannotpenetrate the paper. Furthermore, after application ofthe film of thesecond bathing liquid has been formed on the first prebathing liquid,the first prebathing liquid is prevented from mingling with the liquiddeveloper and thus, deterioration by contamination of the liquiddeveloper bath is prevented.

The first prebathing liquid may be applied only to the opposite ornonimage bearing surface of the electrophotographic sheet instead ofbeing applied to both surfaces.

After treatment with the first prebathing liquid, theelectrophotographic sheet may be subjected to the second prebathingtreatment immediately. This practice, however, is not desirable becausethe first prebathing liquid intermingles with the second prebathingliquid and eventually finds its way gradually into the liquid developer.No serious problem, however, is posed when the volume of the secondprebathing liquid is so large that introduction of a relatively smallvolume of the first prebathing liquid would provide no noticable changein the composition. However, large differences in the relative volumesof the liquids are not normally encountered. Therefore, to insureprevention of mixing of the first prebathing liquid with the liquiddeveloper, it is preferred that subsequent to treatment of theelectrophotographic sheet with a first prebathing liquid that the sheetbe passed between insulating squeezing rollers to remove the portion ofthe first prebathing liquid which may adhere to the surfaces of thesheet. Thereafter, the electrophotographic sheet may be treated with thesecond prebathing liquid.

According to this invention, after development of the electrostaticlatent image present on the electrophotographic sheet with a liquiddeveloper, the sheet may be passed through squeezing rollers to removethe residual liquid developer and may thereafter be easily dried becauseonly the low-boiling liquid remains lodged inside the construction ofthe paper. However, where the developing time is long, partialreplacement may occur between the first prebathing liquid lodged in thepaper and the liquid developer. It is, therefore, desirable to wash offthe liquid developer with a low-boiling cleaning liquid afterdevelopment and at the same time cause the cleaning liquid to replacethe liquid developer which has penetrated deep into the paper. Anysuitable cleaning liquid may be employed. Typically, the cleaning liquidmay be one or more of the liquids used in the first prebathingtreatment.

Any suitable material may be employed as the first prebathing liquid.Typically, the first prebathing liquid is selected to have suchproperties that it does not destroy the electrostatic latent imagepresent on the electrophotographic layer, nor does it attack thestructure of the electrophotographic sheet. Typically, these materialsare selected from the group of nonpolar, highly insulating organicliquids having boiling points below about C. To prevent destruction ofthe electrostatic latent image it is generally preferred to use liquidshaving dielectric constants below about 3.5, resistivities greater thanabout 10 ohm-cm. and solubility parameters below about 8. Typicalspecific materials useful as the first prebathing liquid includecyclohexane, n-hexane, n-heptane, isopentane, CCl FCCl F, CCl FCClF Anysuitable liquid may be employed as the second prebathing liquid.Typically, the second prebathing liquids have boiling points in therange of from about to about 300 C., dielectric constants less thanabout 3.5, volume resistivities greater than 10 ohm-cm. and solubilityparameters below 8. Typical specific materials include kerosene andlsopar H, an isoparaffinic hydrocarbon available from Humble Oil andRefining Company.

From the above description of the invention, the choice of specificmaterials and operating conditions is deemed to be well within the scopeof those skilled in the art and therefore, the scope of the invention isnot limited by the hereinabovementioned illustrative materials. Forexample, while photosensitive paper comprising zinc oxide in aninsulating binder layer coated on ordinary paper which is fibrous andcan be impregnated are discussed above, it is to be understood thatother imaging members may be employed and that the choice of particularimaging member and particular development system may be readilydetermined by one skilled in the art. For example, cadmium sulfide, zincsulfide, zinc selenide, cadmium selenide, titanium dioxide,phthalocyanine and polyvinyl carbazole may be employed as aphotoconductive material. In addition, other suitableelectrostatographic imaging members may be employed.

Development of the electrostatic latent image may be obtained with anysuitable liquid developer. Typical liquid developers containelectroscopic marking particles dispersed in an insulating liquidvehicle and may also contain control agents and suspending agents fortheir well-known functions. The liquid employed must have a relativelyhigh insulating value, generally having a volume resistivity greaterthan about 10" ohm-cm. so as not to affect the electrostatic chargepattern on the insulating layer and low dielectric constants of lessthan about 3.5. Typical specific vehicles include hydrocarbons such asbenzene, xylene, hexane, naptha, kerosene, halogenated hydrocarbons suchas carbon tetrachloride, trichloroethylene and chloroform. Typicalelectroscopic marking particles include among others, charcoal, carbonblack, magnesium oxide, lithopone, cadmium yellow, chrome yellow, cobaltblue, cadmium red, burnt siena, Hansa yellow, rose bengal andphthalocyanine. Typically, the electroscopic marking particles arepresent in an amount of from about 4 to about grams per liter. Theelectroscopic marking particles are conventionally dispersed andsuspended in the liquid by stirring or agitation and where a highlyuniform and stable suspension is desired, the suspension may be passedthrough a colloid mill.

The electrophotographic sheet may be treated with the first or secondprebathing liquid at any suitable time. It may, for example, be treatedsequentially immediately following formation of the electrostatic latentimage. Alternatively, the electrophotographic sheet may be treated firstwith the first prebathing liquid and may even be treated with a secondprebathing liquid prior to the formation of the electrostatic latentimage. The prebathing liquid may be applied to the electrophotographicsheet in any suitable manner. The sheet may, for example, be immersed inthe prebath or may be brought into contact with a sponge or feltimpregnated with a prebathing liquid. The sheet may also be sprayed withthe prebathing liquid. A particularly preferred means of applying theprebathing liquid is by means of a spongy roller impregnated with theliquid which is rolled across the electrophotographic sheet andtransfers theliquid to the sheet on contact.

DESCRIPTION OF PREFERRED EMBODIMENTS The following preferred examplesfurther define, describe and compare preferred materials, methods andtechniques of the present invention. In the examples, all parts andpercentages are by weight unless otherwise specified.

EXAMPLE 1 An electrophotographic sheet is prepared by coating a sheet ofart paper about 150 microns in thickness which is undercoated with aconductive material such as conductive polymer 261 (available fromCalgon Corporation) with a mixture of photoconductive zinc oxide powderin an insulating resin which when dry has a thickness of about 7microns. The electrophotographic sheet is charged negatively in the darkto a surface potential of about 200 volts. The charged sheet is exposedto light projected through an original to form an electrostatic latentimage. The image bearing sheet is then immersed for about 5 seconds inlsopar E, a low-boiling, insulating, isoparaffinic hydrocarbonavailablefrom Humble Oil and Refining Company and subsequently passesthrough a pair of squeezing rollers made of silicon rubber to remove thelsopar E adhering to both surfaces of the sheet. The pores in thefibrous construction of the base paper are impregnatedwith lsopar E.Thereafter, the electrophotographic sheet is soaked for about 5 secondsin the second prebathing liquid or lsopar H, a high-boiling, insulating,iosparaffinic hydrocarbon also available from Humble Oil and RefiningCompany, removed from the bath, held up to allow the second prebath totrickle down and immersed in the liquid developer. The liquid developeris prepared by dispersing carbon black, ranging in particle size of fromabout 0.1 to about 1 micron in lsopar H with a small quantity of varnishdispersion stabilizer, also being present. After about 1 minuteimmersion in the liquid developer, the sheet is removed and passedthrough a pair of squeezing rollers to remove liquid developer adheringto both surfaces of the sheet. The squeezing roller which contacts thesurface carrying the toner image is a hard, smooth metallic roller witha mirror finish surface and the squeezing roller that contacts thereverse soft surface is a soft, resilient rubber roller. Thereafter, thesheet is exposed to a current of warm air at about 50 C. for about 20seconds to completely vaporize the liquid lodged deep in the paper. Adry print with substantially no fogging in the background areas isproduced.

EXAMPLE ll The procedure of Example 1 is repeated except thatimmediately following development of the electrostatic latent image onthe electrophotographic layer, both surfaces of the electrophotographicsheet are sprayed with a cleaning liquid of lsopar E to remove theliquid developer which has adhered to both surfaces. Theelectrophotographic sheet is then passed through the squeezing rollersto remove the cleaning liquid followed by drying. Complete drying isobtained in several tens of seconds when the sheet is allowed to standat normal room temperature. When exposed to a current of warm air at 50C., drying is obtained in less than about 10 seconds. Dry prints ofsimilar quality to those obtained in Example I are produced.

' EXAMPLE in The procedure of Example I is repeated except that prior toformation of the electrostatic latent image on the electrophotographicsheet, the sheet is first contacted with a sponge roller which isimpregnated with the first prebathing liquid, lsopar E. Thereafter, theformation of the electrostatic latent image, the second prebathingtreatment and the remaining procedure of Example 1 are repeated. Printsof quality similar to those obtained in Example I are produced.

Although specific materials and operational techniques are set forth inthe above exemplary embodiments using the development techniques of thisinvention, these are merely intended as illustrations of the presentinvention. There are other developer materials and techniques than thoselisted above which may be substituted for those in the examples withsimilar results. Other modifications of the present invention will occurto those skilled in the art upon a reading of the present disclosure,which modifications are intended to be included within the scope of thisinvention.

What is claimed is:

1. In an electrostatographic imaging method comprising forming anelectrostatic latent image on an electrostatographic imaging membercomprising an image layer capable of receiving an electrostatic latentimage coated on a fibrous substrate, developing said electrostaticlatent image by contacting said imaging layer with a liquid developercomprising marking particles dispersed in an insulating liquid anddrying said imaging member, the improvement comprising prior todevelopment of the electrostatic latent image, contacting at least theside of said imaging member opposite and imaging layer with a firstprebathing liquid comprising an insulating nonpolar organic liquidhaving a boiling point no greater than about C. and subsequently,contacting both surfaces of said imaging member with a second prebathingliquid comprising an insulating organic liquid having a boiling pointbetween about and 300 C.

2. The method of claim 1 wherein both surfaces of said imaging memberare contacted with said first prebathing liquid.

3. The method of claim 1 wherein said imaging member is contacted withsaid first prebathing liquid prior to formation of said electrostaticlatent image.

4. The method of claim 1 wherein said imaging member is contacted withsaid first prebathing liquid after formation of said electrostaticlatent image.

5. The method of claim 1 wherein following development of saidelectrostatic latent image, said imaging layer is contacted with acleaning liquid comprising an insulating liquid having a boiling pointless than about 100 C.

6. The method of claim 1 wherein said imaging layer comprisesphotoconductive pigment particles substantially unifonnly dispersed inan insulating material.

7. The method of claim 6 wherein said photoconductive pigment particlescomprise zinc oxide and said insulating material is a resin.

- 8. The method of claim 1 wherein excess first prebathing liquid isremoved from the surface of said imaging member before contact with saidsecond prebathing liquid.

9. The method of claim 1 wherein said fibrous substrate is paper.

2. The method of claim 1 wherein both surfaces of said imaging memberare contacted with said first prebathing liquid.
 3. The method of claim1 wherein said imaging member is contacted with said first prebathingliquid prior to formation of said electrostatic latent image.
 4. Themethod of claim 1 wherein said imaging member is contacted with saidfirst prebathing liquid after formation of said electrostatic latentimage.
 5. The method of claim 1 wherein following development of saidelectrostatic latent image, said imaging layer is contacted with acleaning liquid comprising an insulating liquid having a boiling pointless than about 100* C.
 6. The method of claim 1 wherein said imaginglayer comprises photoconductive pigment particles substantiallyuniformly dispersed in an insulating material.
 7. The method of claim 6wherein said photoconductive pigment particles comprise zinc oxide andsaid insulating material is a resin.
 8. The method of claim 1 whereinexcess first prebathing liquid is removed from the surface of saidimaging member before contact with said second prebathing liquid.
 9. Themethod of claim 1 wherein said fibrous substrate is paper.